The invention relates to a method for fabricating a photomask, and more particularly, to a method for fabricating a photomask using a polymer.
A semiconductor memory device includes a variety of patterns. These patterns are formed through a photolithography including an exposure process, a development process, and an etching process using a photoresist pattern formed by the photolithography as an etch mask. The exposure process is a process for transferring a pattern on a photomask to a photoresist layer on a wafer. Therefore, if the pattern on the photomask is not formed correctly, a desired photoresist pattern cannot be formed correctly.
Recently, as semiconductor memory devices become more highly integrated, the patterns of the semiconductor memory devices become finer. Therefore, the patterns on the photomasks also become finer. An electron-beam lithography is widely used as a method for forming the patterns on the photomask. To form a phase-shift mask, for example, a phase-shift layer, a chromium film and a resist layer are sequentially formed on a transparent substrate. Then, exposure and development processes are performed on the resist layer using the electron-beam lithography to form a resist pattern. An exposed portion of the chromium film and an exposed portion of the phase shift layer are sequentially removed using the resist pattern as an etch mask to form a phase shift pattern and a chromium pattern. Thereafter, the resist pattern is removed. The chromium pattern is also removed from regions except the frame region.
When the electron beam lithography process is used to form the pattern on the photomask, it is difficult to apply the resist uniformly on an entire surface of a wafer. Non-uniform application of the resist decreases uniformity of a critical dimension (CD) over the locations in the wafer, and thus decreases the productivity of the wafer. In addition, a physical limitation of the lithography equipment imposes limitation on accuracy of forming the pattern, and instability of the lithography process causes a variety of pattern defects. Accordingly, an additional process for correcting the defects is necessarily required. Furthermore, after the process for removing the resist layer, unnecessary impurities tend to remain, which may be a major cause of pattern defects.